The present invention relates to a collapsible filter for introduction into a blood vessel of a patient and of the kind comprising a number of legs diverging from an apical hub and each having a reversely turned hook at its distal end with respect to said hub.
The invention is particularly, but not exclusively concerned with so-called vena cava filters intended for introduction into the vena cava, particularly the inferior vena cava, to entrap thrombi or emboli in the blood flow through the vein and prevent them from reaching the patients lungs and causing pulmonary embolization.
Such filters are known in a variety of different types as disclosed e.g. in U.S. Pat. Nos. 3,540,431, 3,952,747 and 4,619,246.
U.S. Pat. No. 3,540,431 discloses the so-called Mobbin-Uddin or MU-Filter which comprises a skeletal body of spoke shaped members inclined in a generally axial direction and a canopi of filtering media covering the struts and spanning the space between them.
Since the canopy portion has the effect of closing a major part of the vena cava this filter type is no longer much in use.
Another filter type is the so-called Kimray-Greenfield filter disclosed in U.S. Pat. No. 3,952,747, which contain a rather comprehensive analysis of the background and the state of the art. This filter device offers the advantage of a very moderate reduction of the cross-sectional area of the vein combined with a high filter efficiency, but suffers from the disadvantage of requiring a rather high caliper of the insertion instrument.
The latter disadvantage is avoided in the so-called Gunther filter disclosed in U.S. Pat. No. 4,619,246, which comprises a collapsible basket assuming in its expanded state the shape of an apertured elongated solid of revolution with a number of anchoring legs extending from one end of the filter basket.
A disadvantage of the Gunther filter is its considerable axial length, which may cause difficulties in the insertion operation.